In a radio access network of a WCDMA system, a soft handover is an essential feature which, allows an efficient use of the capacity which can be offered by network. In a soft handover state, one mobile station (MS) is connected using a plurality of radio cells contained in its active set (AS) as a resource. The MS monitors levels of received signals broadcasted from neighboring base stations, compares them to a set of thresholds, and reports the measurement of neighbor cells to a radio network controller (RNC) controlling this specific MS, i.e. a Serving RNC (SRNC), and, using this information, the SRNC may transmit a command to the MS to add a new radio cell to its active set, delete a radio cell, replace one or more radio cells, perform an interfrequency handover, and so on. The active set is defined as a set of base stations from which the same user information is sent, simultaneously demodulated and coherently combined.
However, a problem arises when the new or “candidate” cell to be added to the active set is not controlled by the SRNC (such a cell is referred to as “drift cell”), because the load or congestion status of that cell is not available at the SRNC.
Another problem arises when the drift cell is already in the active set, i.e. radio cells which are actively used in the present connection of the MS (inter RCN soft handover). Also in this case, the load status of the drift cell is not known to the SRNC and cannot be considered when the SRNC decides on the execution of a handover, i.e. a handover for traffic reasons is not possible.
A simple solution to the above problems is that the SRNC ignores the load status of the drift cell, and the load status is not used in the decision to add this drift cell to the respective set of the MS. In contrast thereto, the load status of the cells controlled by the SRNC is available and can be used by a handover algorithm performed in the SRNC. When the SRNC decides to add the drift cell to the active set solely on the basis of the measurement report, it transmits a dedicated message (branch addition request) to the so-called Drift RNC (DRNC) controlling the drift cell, wherein the DRNC decides whether to accept or reject the addition of the new cell on the basis of the cell load status (admission control procedure).
However, the load of the drift cell cannot be checked by the SRNC, even if the cell is in the active set of the MS controlled by the SRNC Thus, the radio resource management (RRM) algorithm such as a handover control algorithm of the SRNC cannot be used in an efficient manner, because the load of the drift cell is not known. This leads to the following specific problems:
1) The SRNC may decide to add the drift cell to the active set even if the load of the cell is high and some other radio cells of less load could have been selected. Accordingly, the SRNC sends a branch addition message to the DRNC which may accept the new cell, wherein the increased load of the accepted radio cell may cause an increased interference such that the distribution of the load in the network is not balanced. Otherwise, the DRNC may refuse the new radio cell (branch), wherein the SRNC decides on the following step in accordance with its control algorithms, which causes a delay due to the external signaling between the SRNC and the DRNC. In a worst case scenario, the SRNC may try a view moments later to add the same radio cell to the active set to another or even the same MS.
2) The load of a drift cell already included in the active set of the MS may increase. Since the drift cell is not controlled by the SRNC, the SRNC is not aware of the load increase, which may lead to an unbalanced network. Moreover, the load of the cell may increase to such an extend that the cell reaches a congestion state, and the branch is lost or deleted by the DRNC. Thereby, the quality of the connection may decrease.
3) The SRNC may request to add a drift cell which is not available due to a congestion Status or a hardware failure to the active set of the controlled MS. Again, this might lead to a decrease in the quality of the connection.